'Can I sleep at night? Like a baby,' he said.

Six months ago, he told a local magazine: 'I have shot two women. One had a long history of previous offences, and had killed an infant by packing it full of heroin and trying to carry it across the border to Malaysia. What can we do with people like this?'

He has executed three prisoners this year out of 950 prisoners currently on death row - many for drug offences following the Thai government's much publicised 'war on drugs'. Of those prisoners, 44 have exhausted all appeals and are awaiting execution, although 16 have appealed to King Bhumibol Adulyadej for a royal pardon.

A Corrections Department spokesman said it was unlikely there would be any more executions by gun.

Prisoners executed by machine-gun at the maximum security prison known among inmates as the 'big tiger' are shot with their hands tied but allowed to hold lotus blossoms, joss sticks and a prayer candle if they wish.

They are tied to a wooden stake, behind a blue screen featuring a target so the executioner knows where to aim for a heart shot.

Death-row inmates find out the precise time of their execution only a few hours before it is carried out. Once a pardon is refused by the king, they are offered a last meal, a chance to pray with a monk and write a will, then taken out to be shot.

'Lethal injection allows the condemned to die in peace with no pain or suffering,' said Corrections Department deputy director-general Natthee Jitsawang. He admitted the department had chosen the method after a growing barrage of criticism about the inhumanity and bloody excess of executing prisoners in a hail of machine-gun fire.

One doctor at the prison's hospital, Dr Manop Srisubhanthavorn, said three separate injections would be given.

The first would induce unconsciousness, the second muscular paralysis and the final injection would stop the heart. The process would take less than five minutes and be painless.

Justice Minister Pongthep Thepkanchana has proposed using three separate executioners who would each push a button, without knowing who had actually delivered the drug that brought on cardiac arrest.

Sodium thiopental, an anaesthetic, puts the inmate into a deep sleep, followed by a flushing of the intravenous line with saline solution. Then pancuronium bromide is used to paralyse the diaphram and lungs. After another saline flush, potassium chloride is administered to stop the heart.

In the United States, where 37 of the 38 death-penalty states now use lethal injections, problems have ranged from lack of suitable veins, especially with drug addicts, clogged intravenous lines, and incorrect dosages, which may cause the anaesthetic to wear off, rendering the process far from painless.

Another prison medic, Dr John Lerwitworapong, said he was happy lethal injections were being introduced, as he hated having to check bodies riddled with bullets to make sure they were dead.

'Make no mistake, executions are horrible,' he said. 'I've watched about 20 men shot, and it never gets easier. And sometimes they don't die instantly, even after being hit by several bullets. Sometimes they scream in agony.

'I'm a doctor and a Buddhist, so of course I'm against any kind of execution, but if it comes down to a choice, lethal injection is a far more humane way to go about it.'

Prisoners sentenced to death can appeal to the Court of Appeal, the Supreme Court and then the king, but the king has already warned those on death row because of drugs charges that he is not in a lenient mood.

Of course, death might come as a welcome release to some: in his 1997 book The Damage Done, Australian heroin trafficker Warren Fellows, who served 12 years in Bang Kwang, described it thus: 'It was so unbelievably crowded that if you didn't make it to the yard early in the day there was simply nowhere to sit. Beds were nothing but straw mats, and pillows of any kind were not allowed. The prison was hot with fleas, and I contracted lice within days. Rats ran riot. All the water we had at our disposal came from the local river, which was little more than a stream of muddy red filth ... pumped from a pipe 50 metres downstream from the prison sewage outlet.'

Prison director Pittaya Sanghanakin said conditions had improved since Fellows' time there, with cleaner, less crowded cells.